method for transmitting channel quality information in a multiple input multiple output system

ABSTRACT

A method for transmitting a CQI in a MIMO system is disclosed. A method for allowing a receiver to feed back a CQI value to a transmitter in a Multiple Input Multiple Output (MIMO) system includes, receiving a transmission (Tx) pilot signal for each Tx antenna from a base station (BS), measuring a first CQI value of a first codeword and a second CQI value of a second codeword on the basis of the pilot signal, and transmitting the first CQI value of the first codeword and the second CQI value of the second codeword to the base station (BS), wherein at least one of the first and second CQI values includes specific information capable of indicating a transmission restriction status of a corresponding codeword.

TECHNICAL FIELD

The present invention relates to a Multiple Input Multiple Output (MIMO)system, and more particularly to a method for transmitting a channelquality information (CQI) in a Multiple Input Multiple Output (MIMO)system.

BACKGROUND ART

A Multiple Input Multiple Output (MIMO) technology will hereinafter bedescribed in detail. In brief, the MIMO technology is an abbreviation ofthe Multi-Input Multi-Output technology. The MIMO technology usesmultiple transmission (Tx) antennas and multiple reception (Rx) antennasto improve the efficiency of Tx/Rx data, whereas a conventional art hasgenerally used a single transmission (Tx) antenna and a single reception(Rx) antenna. In other words, the MIMO technology allows a transmittingend or receiving end of a wireless communication system to use multipleantennas (hereinafter referred to as a multi-antenna), so that thecapacity or performance can be improved. For the convenience ofdescription, the term “MIMO” can also be considered to be amulti-antenna technology. In more detail, the MIMO technology is notdependent on a single antenna path to receive a single total message,collects a plurality of data pieces received via several antennas, andcompletes total data. As a result, the MIMO technology can increase adata transfer rate within a specific coverage, or can increase systemcoverage at a specific transfer rate. In other words, the MIMOtechnology is the next-generation mobile communication technologycapable of being applied to mobile communication terminals or relays.

The MIMO technology from among a variety of technologies can greatlyincrease an amount of communication capacity and Tx/Rx performanceswithout allocating additional frequencies or increasing an additionalpower. Due to these technical advantages, most companies or developersare intensively paying attention to this MIMO technology. Thenext-generation mobile communication technology requires a data transferrate higher than that of a conventional mobile communication technology,so that it is expected that the effective MIMO technology is requisitefor the next-generation mobile communication technology. Under thissituation, the MIMO communication technology is the next-generationmobile communication technology capable of being applied to mobilecommunication terminals or relays, and can extend the range of a datacommunication range, so that it can overcome the limited amount oftransfer data of other mobile communication systems due to a variety oflimited situations.

The above-mentioned MIMO technology can be classified into a spatialdiversity scheme and a spatial multiplexing scheme. The spatialdiversity scheme increases transmission reliability using symbolspassing various channel paths. The spatial multiplexing schemesimultaneously transmits a plurality of data symbols via a plurality ofTx antennas, so that it increases a transfer rate of data. Detaileddescriptions of the spatial diversity scheme, the spatial multiplexingscheme, and the combination thereof will hereinafter be described indetail.

Firstly, the spatial diversity scheme will hereinafter be described. Thespatial diversity scheme is classified into a space-time block codescheme and a space-time Trellis code scheme which simultaneously uses adiversity gain and a coding gain. Generally, a bit error ratio (BER)improvement performance and a code-generation degree of freedom of thespace-time Trellis code scheme are superior to those of the space-timeblock code scheme, whereas the calculation complexity of the space-timeblock code scheme is superior to that of the space-time Trellis codescheme. The above-mentioned spatial diversity gain corresponds to theproduct or multiplication of the number (N_(T)) of Tx antennas and thenumber (N_(R)) of Rx antennas, as denoted by N_(T)×N_(R).

Secondly, the spatial multiplexing scheme will hereinafter be described.The spatial multiplexing scheme is adapted to transmit different datastreams via individual Tx antennas. In this case, a receiver mayunavoidably generate mutual interference between data piecessimultaneously transmitted from a transmitter. The receiver removes thismutual interference from the received data using a proper signalprocessing technique, so that it can receive the resultant data havingno interference. In order to remove noise or interference from thereceived data, a maximum likelihood receiver, a ZF receiver, a MMSEreceiver, a D-BLAST, or a V-BLAST may be used. Specifically, if atransmitting end can recognize channel information, a Singular ValueDecomposition (SVD) scheme may be used to remove the noise orinterference.

Thirdly, the combination of the spatial diversity scheme and the spatialmultiplexing scheme will hereinafter be described. Provided that only aspatial diversity gain is acquired, the performance-improvement gain isgradually saturated in proportion to an increasing diversity order.Otherwise, provided that only the spatial multiplexing gain is acquired,a transmission reliability of a RF channel is gradually deteriorated. Asa result, a variety of schemes capable of acquiring all theabove-mentioned two gains simultaneously while solving theabove-mentioned problems have been intensively researched by manycompanies or developers, for example, a double-STTD scheme and aspace-time BICM (STBICM) scheme.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method fortransmitting a channel quality information (CQI) in a Multiple InputMultiple Output (MIMO) system that substantially obviates one or moreproblems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a method fortransmitting CQI in a Multiple Input Multiple Output (MIMO) system.

Another object of the present invention is to provide a method forindicating some codewords in transmission restriction status using CQI,on the condition that several codewords of a MIMO system have beentransmitted from a transmitter to a receiver and the CQI measured by thereceiver in association with each codeword has been transmitted from thereceiver to the transmitter in response to the transmitted severalcodewords. In other words, the receiver informs that CQI of somecodewords cannot be measured.

In brief, the present invention aims to allow the receiver to indicatethat a channel quality of a corresponding codeword has an unavailablereception status.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod for allowing a receiver to feed back a CQI value to a transmitterin a Multiple Input Multiple Output (MIMO) system comprising: receivinga transmission (Tx) pilot signal for each Tx antenna from a base station(BS); measuring a first CQI value of a first codeword and a second CQIvalue of a second codeword on the basis of the pilot signal; andtransmitting the first CQI value of the first codeword and the secondCQI value of the second codeword to the base station (BS), wherein atleast one of the first and second CQI values includes specificinformation capable of indicating a transmission restriction status of acorresponding codeword.

Preferably, the first and second CQI values are indicative of CQI ofsome parts of a total band.

Preferably, the first and second CQI values are transmitted via at leastone of quantized channel status information, a SINR (Signal toInterference plus Noise Ratio), and a MCS (Modulation and CodingSelection) level index.

Preferably, the specific information indicating the transmissiondisallowance information is any one of a SINR (Signal to Interferenceplus Noise Ratio) of ‘−∞dB’, a coding rate of ‘0’, a modulation order of‘0’, and a predetermined MCS level index.

Preferably, the predetermined MCS level index is predetermined toindicate either of the coding rate of ‘0’ or the modulation order of‘0’.

Preferably, the second CQI value includes a relative channel informationvalue associated with the first CQI value.

Preferably, provided that the second CQI value is reconstructed by thefirst CQI value and its associated relative value, if the reconstructedvalue is a non-existing value or is in a transmission restriction statusbased on the first CQI value, the second CQI value indicates thetransmission restriction status.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

ADVANTAGEOUS EFFECTS

The MIMO system according to the present invention can transmit a CQI.According to the following embodiments of the present invention, if theMIMO system transmits several codewords and the CQI of each codeword, itcan indicate that some codewords is in transmission restriction statususing the CQI.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a block diagram illustrating a transmission structure of aMIMO system including the (2×2)-antenna structure;

FIG. 2 is a conceptual diagram illustrating a method for transmittingCQI from a UE to a Node-B;

FIG. 3 shows the CQI result measured by a receiving end according;

FIG. 4 is a flow chart illustrating a method for transmitting the CQIaccording to the present invention; and

FIG. 5 is a conceptual diagram illustrating a Multiple Input MultipleOutput (MIMO) system according to the present invention.

MODE FOR INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Prior to describing the present invention, it should be noted that mostterms disclosed in the present invention correspond to general termswell known in the art, but some terms have been selected by theapplicant as necessary and will hereinafter be disclosed in thefollowing description of the present invention. Therefore, it ispreferable that the terms defined by the applicant be understood on thebasis of their meanings in the present invention.

For the convenience of description and better understanding of thepresent invention, general structures and devices well known in the artwill be omitted or be denoted by a block diagram or a flow chart.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

In the meantime, if several transmission information units areoverlapped and then received, the present invention can expectperformance improvement using an interference-cancellation receiver. Abrief description of the interference-cancellation structure will be asfollows.

According to the interference-cancellation structure, after the firstinformation is demodulated/decoded from a total reception signal inwhich some information is overlapped, information associated with thefirst information is removed from the total reception signal. A secondsignal is demodulated/decoded by the resultant signal having no firstinformation removed from the reception signal. A third signal isdemodulated/decoded by the resultant signal having no first- andsecond-information removed from the first reception signal. A fourthsignal or other signal after the fourth signal repeats theabove-mentioned processes, so that the fourth or other signal isdemodulated/decoded. In order to use the above-mentioned interferencecancellation method, the demodulated/decoded signal removed from thereception signal must have no error. If any error occurs in thedemodulated/decoded signal, error propagation occurs so that a negativeinfluence continuously affects all the demodulated/decoded signals.

As described above, in order to minimize the error propagation caused bythe interference cancellation, it is preferable that the interference isselectively removed after determining the presence or absence of anerror in the demodulated/decoded signal. One of methods for determiningthe presence or absence of the error in each transmission information iscyclic redundancy check (CRC) method.

A general communication system performs encoding of transmissioninformation of a transmitting end using a forward error correction code,and transmits the coded information, so that an error experienced at achannel can be corrected by a receiving end. The receiving enddemodulates a received (Rx) signal, and performs decoding of forwarderror correction code on the demodulated signal, so that it recovers thetransmission information. By the decoding process, the Rx-signal errorcaused by the channel is corrected.

Each of all forward error correction codes has a maximum-correctablelimitation in a channel error correction. In other words, if a reception(Rx) signal has an error exceeding the limitation of a correspondingforward error correction code, a receiving end is unable to decode theRx signal into information having no error. Therefore, the receiving endmust be able to check the presence or absence of an error in the decodedinformation. In this way, a specialized coding process for performingerror detection is required, separately from the forward errorcorrection coding process. Generally, a Cyclic Redundancy Check (CRC)code is used as an error detection code.

The CRC method is an exemplary coding method for performing the errordetection. Generally, the transmission information is coded by the CRCmethod, and then the forward error correction code is applied to theCRC-coded information. A single unit coded by the CRC and the forwarderror correction code is generally called a codeword. In other words, aunit of distinctive information processed by the CRC coding is called acodeword.

Therefore, in order to minimize the error propagation caused by theinterference cancellation, it is preferable that the interference isselectively removed after determining the presence or absence of anerror in the demodulated/decoded signal. Therefore, a representativemethod for using the interference cancellation technology is a specificcase in which several transmission information segments and severalcodewords are used.

If several transmission information units are overlapped and thenreceived, the present invention can expect performance improvement usingan interference-cancellation receiver. There are many cases in theabove-mentioned case in which several transmission information segmentsare overlapped and then received, for example, a case in which the MIMOtechnology is used, a case in which a multi-user detection technology isused, and a case in which a multi-code technology is used. In order touse the above-mentioned interference cancellation technology, severaltransmission information segments must be overlapped/transmitted viamultiple antennas. In other words, if the spatial multiplexingtechnology is used, each of transmission information is detected, and atthe same time the interference cancellation technology can be used.

In this case, several transmission information segments may be definedas several codewords as described above, and be then transmitted. Thefollowing embodiment assumes that the MIMO system transmits a pluralityof codes and transmits control information per each codeword. However,the following embodiment has been disclosed for only illustrativepurposes, and it should be noted that the scope and spirit of thisembodiment will be equally applied to various transmission informationconstituent units of various systems capable of transmitting severaltransmission information segments.

FIG. 1 is a block diagram illustrating a transmission structure of aMIMO system including the (2×2)-antenna structure.

Referring to FIG. 1, the system capable of transmitting a plurality ofcodewords, each of which is used as the aforementioned CRC attachmentunit, is able to construct several transmission chains as many as thenumber of codewords capable of being simultaneously transmitted. FIG. 1shows an example of the transmission structure equipped with the(2×2)-antenna structure. In the case of min(M,N)=2, the maximum numberof transmittable streams (i.e., a maximum rank) may be set to “2”.Referring to FIG. 1, the current rank is 2. If a CW1 (10) and a CW2 (12)are transmitted, the MIMO system according to the present invention canrecognize that a transmission chain is constructed. CRCs are attached toCW1 (10) and CW2 (12) as denoted by ‘12’ and ‘13’. The channel coding iscarried out on each of the CRC attachment results 12 and 13. Themodulated streams 15 and 17 are precoded by the precoding unit 18, suchthat the precoded result can be transmitted via each Tx antenna 19.

Next, control information transmitted from the above-mentioned MIMOsystem will hereinafter be described in detail. For example, CQI may beused as the above-mentioned control information transmitted from theMIMO system. Namely, a receiving end of the MIMO system measures thestatus of a channel through which a signal is transmitted from atransmitting end, and then deliver information about the status to thetransmitting end.

FIG. 2 is a conceptual diagram illustrating a method for transmittingCQI from a user equipment (UE) to a Node-B.

Referring to FIG. 2, the UE 21 receives a downlink signal from theNode-B 20, and measures a downlink channel status. The UE 21 transmits achannel status measurement result as the CQI back to the Node-B 20.Specifically, the Node-B may be able to perform a link adaptationprocess, such that it can use a maximum amount of channel capacity ofthe mobile communication system and can effectively transmit data tousers. It is preferable that the UE feeds back CQI to the Node-B toperform the link adaptation process.

In the meantime, in multi-carrier system, channel qualities aredifferent per each of frequency bands for data transmission. In order toeffectively allocate resources, the user (i.e., the UE) transmits CQI ofa total frequency band. Therefore, the total frequency band is dividedinto several units of frequency bands, and the CQI can be transmittedvia each unit of frequency bands.

This CQI may be generated in various ways, for example, a method ofsimply quantizing a channel status without any change, a method ofcalculating a SINR (Signal to Interference+Noise Ratio), and a method ofusing MCS (Modulation and Coding Selection) level information toindicate the status of a channel in real condition.

A method of generating CQI based on MCS among various CQI generationmethods will be explained hereinafter. An example for this method is aCQI generation method for a High Speed Downlink Packet Access (HSDPA)transmission scheme under the 3GPP. In case that the CQI is generated onthe basis of MCS, MCS consists of a modulation scheme, a coding scheme,and the resultant coding rate. Hence, it is preferable that at least oneCQI be transmitted per each codeword which is considered as amodulation/coding unit, because the CQI is to be changed according tothe change of the modulation scheme and the coding scheme.

In addition, different channel measurement scheme and/or differentreporting scheme may be applied to CQI according to the type of a signalor a channel. For example, a communication channel between the Node-Band the UE can be generally classified into a data traffic channel and acontrol channel for controlling the data traffic channel. If the datatraffic channel and the control channel have different frequency/spatialbands, the data traffic channel and the control channel may also havedifferent CQI values.

Generally, for a control channel in the multiple-carrier system,frequency diversity and spatial diversity are used throughout the wholeband. Therefore, the CQI for a control channel is measured and fed backfor the whole band.

To the contrary, in case of the data traffic channel, scheduling andspatial multiplexing is performed for each frequency band. Therefore, itis preferable to divide the frequency band into sub-frequency bands andmeasure CQI value for each sub-frequency band, and then feed back themeasured CQI.

Control information transmitted from the MIMO system may be exemplifiedby rank information. The rank information is a control information thatindicates how many independent data streams can be transmitted at acurrent transmission time, when the MIMO system transmits severalindependent data streams. That is, rank is defined as the number ofmaximum data streams that can be transmitted at a certain transmissiontime. Rank may also be called as a spatial multiplexing rate. Rank mightbe decided in consideration of the combination of antennas of atransceiver. For example, the system including M number of Tx antennasand N number of Rx antennas has a maximum rank of min(M,N).

For another example, Tx control information of the MIMO system may beprecoding matrix index information. A MIMO system using a precodingscheme can transmit control information associated with either aprecoding vector or a precoding matrix which is the most appropriate fora current channel status.

The precoding vector or the precoding matrix can be directly deliveredby transmitting control information including configuration informationof a vector or a matrix. Otherwise, on the condition that a plurality ofprecoding matrix are predefined beforehand with a form of codebook, Theprecoding vector or the precoding matrix can be directly delivered bytransmitting index information in corresponding codebook. In this case,the codebook may be predetermined and stored in thetransmission/receiving ends for each rank, or may also be configured inthe form capable of being applied to several ranks and be then stored inthe transmission/receiving ends for each rank. In this way, in the caseof using the above-mentioned codebook, only index information of thepredetermined precoding vector or precoding matrix may be transmitted tothe transmission/receiving ends, such that the transmission load of thecontrol information can be reduced.

The control information including the CQI can be transmitted to an upperlayer signal or a physical layer control signal. In the case oftransmitting the control signal to the physical layer control signal, ifa downlink shared channel (DL-SCH) exists for a UE, then the controlsignal can puncture data symbols or bits of the DL-SCH to betransmitted. Otherwise, the control signal can be transmitted via adedicated control channel such as Physical Uplink Control Channel(PUCCH).

In order to reduce an uplink feedback load, time period and measurementfrequency band may be differently configured for control informationsuch as rank information and precoding information. For example,considering a feedback period from the receiver to the transmitter, rankis insensitive to a time variation whereas CQI is sensitive to a timevariation, such that a transmission period of rank information may beset relatively longer than that of CQI.

FIG. 3 shows a result of generating CQI by measuring channel informationat a receiving end.

FIG. 3 shows the CQI of the control channel 31, the CW1 32 of the datatraffic channel and the CQI of the CW2 33. As described above, when theCQI of the control channel and the data traffic channel is transmitted,the CQI may be configured in different ways for each of the controlchannel and the data traffic channel.

In other words, for the frequency band for measurement, generally,because a control channel is evenly distributed to whole bands, the CQIcan be configured for the whole bands. In case of data traffic channel,frequency bands may be divided into a plurality of CQI sub-bands forscheduling of frequency bands, and then CQI for each of CQI sub-bands 30or a group of CQI sub-bands can be configured.

Because the control channel is transmitted using a spatial diversityscheme and the data traffic channel is transmitted using a spatialmultiplexing scheme, each of data and control information is transmittedthrough different physical channels. As a result, the SINR received at areceiving end may be different to each other as shown in FIG. 3. Inaddition, the SINR for the data traffic channel received at a receivingend may be different according to rank. For example, for rank 2, twocodewords (i.e., a CW1 32 and CW2 33) is transmitted, such that SINR1and SINR2 might decrease due to unexpected interference between CW1 32and CW2 33. Furthermore, provided that at least one CQI is transmittedfor each codeword, the CQI amount fed back may be changed according towhether the rank is 1 or 2, and signaling structure may have differentformat according to the rank.

In the meantime, rank information is generally measured with a unit ofbandwidth greater than the total band or CQI sub-band. However, ifchannel status fluctuates drastically with frequency bands, the rankinformation measured as above may not be fit for some sub-bands. Thatis, provided that a rank value is set to 2 for a whole bandwidth, thechannel status of CW1 32 or CW2 33 in a certain band 34 may be poor asshown in FIG. 3, and thus transmitting only one stream may be moreeffective than transmitting both of two streams.

In the meantime, when feeding back from a receiver to a transmitter, itis effective to transmit rank information with relatively longer periodcompared to the transmitting period for CQI, in consideration of asensitivity to time variation. If the transmission period of the rankinformation is longer than that of the CQI, a pre-reported rankinformation may not match to the rank status at a CQI report time due totime variation. That is, even though it was reported that a rank was 2,a channel status may be changed to a poor channel status, such that itis more preferable that single-codeword transmission be better thantwo-codeword transmission.

Under the above-mentioned two situations, the Node-B may expectreceiving a report on CQI with rank 2 from UE. In other words, theNode-B may expect that two CQI values (CW1 32 and CW2 33) will bereported, and then the Node-B may recognize control informationaccording to the signaling format associated with the expectation.However, if there is a CQI sub-band 34 in which only one stream is to betransmitted, a method for indicating this situation is required.

In the present embodiment according to the present invention, CQIincludes transmission restriction status information which is capable ofindicating not to use a specified codeword for transmitting data. Inaddition, a request to adaptively change the number of transmissioncodewords for some sub-bands may be delivered with the same format.

Based on the control information according to the present embodiment,the transmitting end may select one of following schemes. First, onlyone codeword may be transmitted. Second, for a codeword requested torestrict for transmission with the transmission restrictions statusinformation, the codeword may still be transmitted enduring a highererror rate than a target error rate. Third, for a codeword requested torestrict for transmission with the transmission restrictions statusinformation, the codeword may be transmitted with increased transmissionpower.

FIG. 4 is a flow chart illustrating a method for transmitting the CQIaccording to the present invention.

Referring to FIG. 4, it is provided that rank is set to 2, the whole ofa data traffic channel is divided to a plurality of CQI sub-bands, andCQI reports are performed for each of the CQI sub-bands.

Referring to FIG. 4, the Node-B transmits a pilot signal to each Txantenna at step S40. The UE receives the pilot signal channeltransmitted through according to link adaptation scheme etc., andmeasures Tx channel qualities of two codewords (i.e. CW1 and CW2) on thebasis of the received pilot signal according to a channel estimationscheme. In this case, if channel quality of a specific CQI sub-band isequal to or less than a reference value, CQI including information thatis capable of reporting transmission restriction status is transmittedat S42 sot as to report the above situation to the base station.

In the case of transmitting the CQI, the UE is able to transmit CQIvalues of two codewords as described above. In this case, informationindicating transmission restriction status might be included in the CQIfor the total band or in the CQI for some sub-bands, and then the CQIfor the total band or in the CQI for some sub-bands might betransmitted. For example, provided that a current rank is set to 2 totransmit two codewords (i.e. CW1 and CW2), transmit status informationindicating that only one of the two codewords (CW1 and CW2) can betransmitted may be transmitted as CQI.

So as to selectively limit transmission of codewords capable of beingtransmitted at a current rank, transmission restriction status might beincluded in the CQI for all codewords, otherwise, the transmissionrestriction status might be included in the CQI for specific codewordsin order to restrict a transmission for part of codewords.

In the case of including the transmission restriction status into theCQI for specific codewords, a low-performance codeword might bedetermined as a codeword capable of transmitting the above transmissionrestriction status information in consideration of a statistical channelstatus. In this case, if the low-performance codeword is changed toanother codeword according to a channel-status variation, the codewordcapable of transmitting the transmission restriction status informationcan also be adaptively changed.

Some codewords may also be determined as a dedicated codeword for use oftransmitting transmission restriction status information. Although somecodewords are dedicated in this way, interference that may be causedbetween above codewords can be prevented, therefore, the effectsanticipated by the present invention can be obtained.

Transmission restriction status may be represented in various waysaccording to transmitted CQI value. For example, if a SINR value istransmitted as the CQI, the value of “SINR=−∞dB” may represent thetransmission restriction status. If a code rate or a modulation schemeis transmitted as the CQI value, “code rate=0” or “modulation order=0”may represent the transmission restriction status. Specifically, if aMCS index is transmitted as the CQI, a specific index may be transmittedto represent the transmission restriction status. According to theabove-mentioned methods, the MIMO system matches the combination of bitstransmitted as the CQI with a single logical status, and transmits thematched result.

The table 1 shows a MCS table including transmission restriction statusin a case that MCS index is transmitted as CQI.

TABLE 1 CQI value (state) for CW Function 0 Transmission restrictionstatus(Tx off) 1 Modulation: QPSK, code rate: 1/3 2 Modulation: QPSK,code rate: 1/2 3 Modulation: QPSK, code rate: 2/3 4 Modulation: 16QAM,code rate: 1/3 5 Modulation: 16QAM, code rate: 2/3 6 Modulation: 64QPSK,code rate: 1/3 7 Modulation: 64QPSK, code rate: 2/3

As can be seen from Table 1, when the MCS index is transmitted as CQI,index information of the MCS table may be transmitted as a CQI value ofeach codeword. Table 1 exemplarily shows that the index 0 represents atransmission restriction status. Namely, if the receiving end (e.g., theUE) transmits the index of 0 as the CQI, this situation may berecognized as a transmission restriction status according to anagreement engaged between the transmission/receiving ends.

If the CQI is transmitted as the MCS index, the following table 2 showsanother MCS table including a transmission restriction status accordingto this embodiment.

TABLE 2 CQI value (state) for CW Function 0 Modulation: 0 and/or coderate: 0 1 Modulation: QPSK, code rate: 1/3 2 Modulation: QPSK, coderate: 1/2 3 Modulation: QPSK, code rate: 2/3 4 Modulation: 16QAM, coderate: 1/3 5 Modulation: 16QAM, code rate: 2/3 6 Modulation: 64QPSK, coderate: 1/3 7 Modulation: 64QPSK, code rate: 2/3

The above-mentioned embodiment of Table 2 is similar to that of Table 1.However, in this case, a modulation order 0 and/or a coding rate 0represents a transmission restriction status

Table 3 is an another example of MCS table including a transmissionrestriction status according to this embodiment when a MCS index istransmitted as CQI.

TABLE 3 CQI value CQI value (state) for (state) for CW1 Function CW2Function 0 Modulation: QPSK, 0 Tx off code rate: 1/3 1 Modulation: QPSK,1 Modulation: QPSK, code rate: 1/2 code rate: 1/3 2 Modulation: QPSK, 2Modulation: QPSK, code rate: 2/3 code rate: 1/2 3 Modulation: 16QAM, 3Modulation: QPSK, code rate: 1/3 code rate: 2/3 4 Modulation: 16QAM, 4Modulation: 16QAM, code rate: 1/2 code rate: 1/3 5 Modulation: 16QAM, 5Modulation: 16QAM, code rate: 2/3 code rate: 2/3 6 Modulation: 64QPSK, 6Modulation: 64QPSK, code rate: 1/3 code rate: 1/3 7 Modulation: 64QPSK,7 Modulation: 64QPSK, code rate: 2/3 code rate: 2/3

In table 3, it is configured sot that the transmission restrictionstatus information can be transmitted by only CQI values of somecodewords.

According to the embodiments of tables 1 and 2, the table 1 and table 2might be applied identically to all codewords, such that thetransmission restriction status information can be transmitted to allcodewords. However, according to the embodiment of Table 3, in the caseof transmitting several codewords (e.g., 2 codewords), different tablesare applied to two codewords. As a result, the transmission restrictionstatus can be transmitted only for part of codewords, e.g. only for CW2.

Although index 0 represents the transmission restriction status in table1 to table 3, it should be understood that other indexes may also beused to indicate the transmission restriction status. In addition,although MCS index is transmitted as CQI in table 1 to table 3, itshould be understood that other information may also be used as CQI.

In the meantime, when CQI is transmitted, more specifically, when CQI ofseveral codewords are transmitted, the MIMO system selects some of theseveral codewords, a specific CQI among all CQIs may be selected as areference. In this case, for other CQIs except the specific CQI, adifference value (δCQI) to the reference value might be transmitted.

For example, if the Node-B transmits two codewords, CQI of CW1 may beused as a reference. In this case, for the CW2, a relative value to thereference value may be transmitted. Provided that a MCS index istransmitted as a CQI and the MSC table of Table 1 is used, when measuredCQI_(CW1)=6 for CW1 and measured CQI_(CW2)=5 for CW2, a value of ‘6’which represents a index value according to the CQI_(CW1)=6 may betransmitted for CW1, and a value of ‘1’ which represents a index valuedifference between the CW1's CQI index and the CW2's CQI index, i.e.,−CQI_(CW2)+CQI_(CW1)=−5+6=1 may be transmitted for CW2.

According to the above-mentioned CQI transmission scheme, if severalcodewords have similar CQI values and there is a little differencebetween the CQI values, transmission of an differential CQI can reducemany more bits than in transmission of the actual value on the basis ofthe reference value, resulting in the reduction of Tx overhead of uplinkcontrol information.

For example, in Table 1, 3 bits are required to transmit the CQI index.Therefore, if there are two codewords, 6 bits are consumed. However,provided that the range of the differential value is established andtransmitted in four steps, for a codeword for transmitting the CQI indexwith the differential, only 2 bits are required. Therefore, a total of 4bits are consumed to transmit two CQI bits, such that 1 bit can bereduced.

The transmission method above using the CQI-differential value may beapplied into the method for transmitting the CQI including transmissionrestriction status information. For example, as previously presumed,provided that the range of the differential value has 4 steps,differential values of the four steps are set to“−CQI_(CW2)+CQI_(CW1)={−2, −1, 0, 1}”, and one of the differentialvalues is transmitted as a CQI signal for CW2, the CQI-index of CQI2 maybe determined one of “CQI_(CW2)+CQI_(CW1)+{−1, 0, 1, 2}”.

When the CQI of CW2 is transmitted using a differential value to thereference CQI, the Node-B can determine (backtrack) the CQI-index of CW2using both the CQI index for transmitting the CW1 and the relative CQIvalue for transmitting the CW2. For example, if CQI_(CW1) is zero (i.e.,CQI_(CW1)=0), CQI_(CW2) may be determined as one of {−1, 0, 1, 2}. IfCQI_(CW1) is ‘1’ (i.e., CQI_(CW1)=1), CQI_(CW2) may be determined as oneof {0, 1, 2, 3}.

When using this backtracking scheme, determined CQI of CW2 may indicatea index value defined as transmission restriction status or anon-existing value which is not defined in the original MCS table. Forexample, the CQI index of CW2 may indicate 0 indicating a transmissionrestriction status of Table 1, or may indicate an index value of −1which is not defined in the index range. If the CQI of CW2 backtrackedusing this scheme indicates a index value defined as transmissionrestriction status or a non-existing value which is not defined in theoriginal MCS table, a Node-B may interpret it as transmissionrestriction status occurred. That means, even when using the scheme ofusing the CQI-differential value, transmission restriction status can bedelivered to the Node-B.

The CQI value for transmitting the CW2 codeword is not limited to anyone of {−1, 0, 1, 2}, but may be defined by various values {−2, −1, 0,1} and {0, 1, 2, 3}. The range of the differential value may be reducedby 1 bit, such that the CQI value may be transmitted with {0, 1}, or{−1, 0}. The transmission-value or range of the differential value,i.e., the number of bits of the differential value, can be adaptivelychanged according to channel status.

FIG. 5 is a conceptual diagram illustrating a Multiple Input MultipleOutput (MIMO) system according to the present invention.

Referring to FIG. 5, a transmission (Tx) signal 50 is coded, modulated,weighted, precoded, and mapped by the signal processor 51 of thetransmitting end, and than the resultant Tx signal is transmitted viaseveral antennas. In this case, the Tx signal is composed of severalcodewords according to a predetermined rank, such that it can bedifferently processed according to the individual codewords. A reception(Rx) signal is processed at a signal processor 52 by inverselyprocessing the process of the transmitting end.

A downlink CQI is measured with the received signal and then transmittedto the transmitting end. In this case, the channel quality may becomposed of various-formatted CQI values, and be then transmitted to thetransmitting end. If several codewords are transmitted according to thisembodiment of the present invention, CQI of each codeword may betransmitted with containing transmission restriction status ifnecessary. A detailed description thereof is similar to those explainedin above.

The transmitting end receives the CQI, processes the Tx signal accordingto the received CQI, and transmits the processed result to the receivingend, such that it can adapt to varying channel condition. In this case,if the transmitting end receives specific information from the receivingend indicating that some codewords are in transmission restrictionstatus, it may transmit only some transmittable codewords irrespectiveof the receiver's feedback rank. Also, the Node-B may change a currentrank to another rank, and may transmit signals via the changed rank.

The above-mentioned CQI may be referred to as channel informationindicator or channel status information, etc. The above-mentioned CQI isnot limited to the above-mentioned terms, but it can also be calledother terms as necessary.

The following embodiments of the present invention will be disclosed onthe basis of a data communication relationship between the Node-B andthe user equipment (UE).

In this case, the Node-B is used as a terminal node of a network viawhich the Node-B can directly communicate with the user equipment (UE).Specific operations to be conducted by the user equipment (UE) in thepresent invention may also be conducted by an upper node of the Node-Bas necessary. In other words, it will be obvious to those skilled in theart that various operations for enabling the Node-B to communicate withthe user equipment (UE) in a network composed of several network nodesincluding the Node-B will be conducted by the Node-B or other networknodes other than the Node-B. The term “Node-B” may be replaced with afixed station, base station (BS), eNode-B (eNB), or an access point asnecessary. The term “user equipment (UE)” may be replaced with a mobilestation (MS) or a mobile subscriber station (MSS) as necessary.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered to be optional factors on thecondition that there is no additional remark. If required, theindividual constituent components or characteristics may not be combinedwith other components or characteristics.

Also, some constituent components and/or characteristics may be combinedto implement the embodiments of the present invention. The order ofoperations to be disclosed in the embodiments of the present inventionmay be changed to another. Some components or characteristics of anyembodiment may also be included in other embodiments, or may be replacedwith those of the other embodiments as necessary.

It is obvious to those skilled in the art that the above embodiments maybe constructed by combining claims having no explicit citation relationsor new claims may also be added by the amendment to be made after thepatent application.

It should be noted that most terminology disclosed in the presentinvention is defined in consideration of functions of the presentinvention, and can be differently determined according to intention ofthose skilled in the art or usual practices. Therefore, it is preferablethat the above-mentioned terminology be understood on the basis of allcontents disclosed in the present invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the MIMO system according to thepresent invention can transmit CQI.

According to the following embodiments of the present invention, if theMIMO system transmits several codewords and the CQI of each codeword, itcan indicate that some codewords are in a transmission restrictionstatus using the CQI.

1. A method for feeding back CQI (channel quality information) to from areceiver to a transmitter in a Multiple Input Multiple Output (MIMO)system comprising: receiving pilot signal from a base station (BS), thepilot signal being transmitted per each transmission antenna; measuringa first channel quality for a first codeword and a second channelquality for a second codeword on the basis of the pilot signal; andtransmitting a first CQI for the first codeword and a second CQI for thesecond codeword to the base station (BS), wherein, at least one of thefirst CQI and the second CQI includes information indicating atransmission restriction status for a corresponding codeword.
 2. Themethod according to claim 1, wherein the first CQI and the second CQIare indicative of CQI of some parts of a total band.
 3. The methodaccording to claim 1, wherein the first CQI and the second CQI aretransmitted in a form of one of quantized channel status information, aSINR (Signal to Interference plus Noise Ratio), and a MCS (Modulationand Coding Selection) level index.
 4. The method according to claim 1,wherein the information indicating the transmission restriction statusis one of a SINR (Signal to Interference plus Noise Ratio) of ‘−∞dB’, acoding rate of ‘0’, a modulation order of ‘0’, and a predetermined MCSlevel index.
 5. The method according to claim 4, wherein thepredetermined MCS level index is predetermined to indicate either of thecoding rate of ‘0’ or the modulation order of ‘0’.
 6. The methodaccording to claim 1, wherein the second CQI includes a relative valuewhich is relatively determined compared to a value of the first CQI. 7.The method according to claim 6, wherein: provided that the second CQIis reconstructed by the value of the first CQI and the relative value,if a value of the reconstructed second CQI is indicative of anon-existing value or the transmission restriction status based on thefirst CQI, the second CQI indicates the transmission restriction status.